A pivotal issue for patients with multiple sclerosis (MS) is the long-term survival and integrity of neurons and oligodendrocytes, and maintenance of the myelin sheath. Over time, the progressive nature of MS results in a chronic disease course with permanent axonal damage, neuronal and oligodendrocyte loss, demyelination, and diminished re-myelination. Studies in animal models have proved useful in exploring disease mechanisms and in testing therapeutic paradigms, however model systems using cells of human origin are clearly essential for the identification of proteins required to maintain and enhance myelination in humans. In this proposal, our goal is to use our human co-culture system of dorsal root ganglia and oligodendrocyte precursor cells (OPC) to determine the extent to which factors proposed to enhance myelination in rodents function similarly in the human system. We will expand upon our preliminary data that show that growth-arrest specific protein 6 (gas6) enhances OPC maturation and axon ensheathment, and test whether in combination with gas6, the activation of cannabinoid receptors, or the retinoid X receptor (RXR) can promote further the maturation and myelinating activity of human OPC in vitro. In Specific Aim 1, we will continue to examine the ability of gas6 to enhance axonal ensheathment, and examine by immunofluorescent microscopy and electron microscopy the extent to which myelin proteins are expressed in the developing myelin sheath, whether compact myelin is achieved over time, and the correlation between axonal diameter and myelin sheath formation. In Specific Aim 2A, we will examine the contribution of agonists and antagonists of cannabinoid (CB) receptors plus and minus gas6 to determine whether CB2 agonists enhance myelination and aid in myelin compaction. In Specific Aim 2B, we will determine whether 9-cis-retinoic acid signaling through the RXR? receptor enhances myelination. These studies will help to define factors required for human myelination. Analysis of growth factors, receptor agonists and signaling molecules will determine the potential role of these factors in enhancing human myelination with a focus on the factors required for compact myelin formation in a human model system.